GroEL, a molecular chaperonin from E. coli mediates folding of many proteins in vivo and in vitro in an ATP driven reaction. GroEL is a tetradecamer comprised of 14 identical subunits arranged as two stacked rings with a seven fold axis of symmetry and a central cavity. It appears to bind early folding intermediates, preventing their aggregation; in the presence of MgATP and a co-chaperonin, GroES, the bound polypeptides are released in a stepwise manner, associated with folding to the native state. The molecular mechanism and energetics of the folding reaction are not known at present. The binding of a variety of nucleotides to GroEL triggers gross conformational changes in the molecule. In many cases this binding event is sufficient to dissociate the polypeptides from the binary complex. We chose to study MgADP binding to GroEL by means of the isothermal titration calorimetry (ITC) to characterize energetics of the binding and of the concomitant allosteric transition. In addition, both tetradecameric GroEL and heptameric GroES provide interesting systems to study subunits oligomerization as they were shown, at least under certain conditions, to be in dynamic equilibrium with monomeric species. The physiological role of low stability of GroEL and GroES oligomers in the absence of ligands is not known but it may be important in regulation of their interactions with partially folded proteins. The energetics of subunits interactions and the effects of such ligands as nucleotides and partially folded polypeptides will be characterized primarily by means of differential scanning calorimetry.